1. Field of the Invention
The present invention relates to a structure of a field effect transistor (FET) necessitating a very high speed operation.
2. Related Background Art
Conventionally, as a very high speed device of this kind, for example, a first HEMT (high electron-mobility transistor) is introduced as shown in FIG. 1. An InP semiconductor substrate 1 is formed thereon with an undoped InP layer 2. The undoped InP layer 2 is formed thereon with an n-Al.sub.x In.sub.1-x As layer 3 on which donors are selectively added to Al.sub.x In.sub.1-x As. The n-Al.sub.x In.sub.1-x As layer 3 is formed thereon with an n.sup.+ -InGaAs layer 4, a gate electrode 5 is formed in Schottky contact with the n-Al.sub.x In.sub.1-x As layer 3 exposed at a recess formed at a center portion, and ohmic electrodes 6 and 7 are formed on the n.sup.+ -InGaAs layer 4.
There is also a second HEMT having the same structure as the first HEMT but made of different composite materials. In the second HEMT, a GaAs semiconductor substrate is used instead of the InP semiconductor substrate 1, and an undoped GaAs layer, an n-AlGaAs layer, and an n.sup.+ -GaAs layer are laminated respectively instead of the undoped InP layer 2, the n-Al.sub.x In.sub.1-x As layer 3, and the n.sup.+ -InGaAs layer 4.
There is also a third HEMT with a structure as indicated in FIG. 2. Namely, an InP semiconductor substrate 11 is formed thereon with an undoped AlInAs layer 12. On the undoped AlInAs layer 12 is further formed an undoped In.sub.y Ga.sub.1-y As layer 13, and on this undoped In.sub.y Ga.sub.1-y As layer 13 is formed an n-Al.sub.x In.sub.1-x As layer 14 in which donors are selectively added to Al.sub.x In.sub.1-x As. Further on the n-Al.sub.x In.sub.1-x As layer 14 is formed an n.sup.+ -InGaAs layer 15, and a gate electrode 16 is formed in Schottky contact with the n-Al.sub.x In.sub.1-x As layer 14 exposed at a recess formed at a center portion, and ohmic electrodes 17 and 18 are formed on the n.sup.+ -InGaAs layer 15.
In addition, there is a fourth HEMT having the same structure as the third HEMT but with different composite materials. In the fourth HEMT, a GaAs semiconductor substrate is used instead of an InP semiconductor substrate 11, and an undoped GaAs layer, an undoped In.sub.y Ga.sub.1-y As layer and an n-Al.sub.x Ga.sub.1-x As layer are laminated respectively instead of the undoped AlInAs layer 12, the undoped In.sub.y Ga.sub.1-y As layer 13, and the n-Al.sub.x In.sub.1-x As layer 14. The donors are selectively added to this n-Al.sub.x Ga.sub.1-x As layer. Further an n.sup.+ -InGaAs layer is used instead of the n.sup.+ -InGaAs layer 15.
However, as in the conventional first HEMT of the prior art hereinbefore described, for a system using heterojunction of AlInAs/InP, electrons travel in an InP layer being a channel and such electrons often produce a real space transfer for making transition to the AlInAs layer disposed in the upper layer of the InP layer. This real space transfer may be explained as follows. An energy band indicated in FIG. 3 is formed at a heterojunction portion of the n-AlInAs layer 3 and the undoped InP layer 2, and the two-dimensional electron gas is accumulated at the oblique-lined portion of the drawing. However, if a high electric field is applied across a drain and a source and the energy of the two-dimensional electron gas becomes higher, the electrons in the two-dimensional electron gas make transition to the n-AlInAs layer 3 as shown by the arrow mark in the drawing.
Generally, a high electric field is applied across a drain and a source during its operation, and as carrying characteristic of electrons is inferior in the AlInAs layer as compared to the InP layer, when this real space transfer occurs the high-frequency characteristic of the FET becomes degraded.
As in the second HEMT of the prior art hereinbefore described, for a system using the heterojunction of AlGaAs/GaAs, electrons travel in an GaAs layer which is to become a channel and such electrons sometimes produce a real space transfer for making transition to the AlGaAs layer disposed in the upper layer of the GaAs layer. For example, this real space transfer may be explained as follows. An energy band in FIG. 4 is formed at the heterojunction portion of the n-AlGaAs layer and the undoped GaAs, and the two-dimensional electron gas is accumulated at the oblique lined portion of the drawing. However, if a high electric field is applied across a drain and a source and the energy of the two-dimensional electron gas becomes higher, the electrons in the two-dimensional electron gas are transferred to the n-AlGaAs layer as shown by the arrow mark in the drawing.
Generally, a high electric field is applied across a drain and a source during its operation, and as a carrying characteristic of electrons is inferior in the AlGaAs layer as compared to the GaAs layer, thus when this real space transfer occurs it deteriorates the high-frequency characteristic of the FET.
In addition, as in the third HEMT of the prior art hereinbefore described, even in a system using the heterojunction of AlInAs/InGaAs, electrons travel in an InGaAs layer 13 which is to become a channel and such electrons sometimes produce real space transfer for making transition to the AlInAs layer 14 disposed in the upper layer of the InGaAs layer 13. This real space transition may be explained as below. An energy band in FIG. 5 is formed at the heterojunction portion of the n-AlInAs layer 14 and the undoped InGaAs layer 13, and the two-dimensional electron gas is accumulated at the oblique lined portion of the drawing. However, if a high electric field is applied across a drain and a source and the energy of the two-dimensional electron gas becomes higher, the electrons in the two-dimensional electron gas are transferred to the n-AlInAs layer 14 as shown by the arrow mark in the drawing.
Generally, a high electric field is imposed across a drain and a source during its operation, and because a carrying characteristic of electrons in the AlInAs layer is inferior to that of the InGaAs layer, when this real space transfer occurs, it deteriorates the high-frequency characteristic of the FET.
Also as in the fourth HEMT of the prior art hereinbefore described, even for a system using the hetero/junction of AlGaAs/InGaAs, electrons travel in an InGaAs layer which will be a channel and such electrons sometimes produce real space transition for transferring to the AlGaAs layer disposed in the upper layer of the InGaAs layer. This real space transfer can be explained as follows. An energy band in FIG. 6 is formed at the hetero/junction portion of the n-AlGaAs layer and the undoped InGaAs layer, and the two-dimensional electron gas is accumulated at the oblique lined portion of the drawing. However, if a high electric field is applied across a drain and a source and the energy of the two-dimensional electron gas becomes higher, the electrons in the two-dimensional electron gas make transition to the n-AlGaAs layer as shown by the arrow mark in the drawing.
Generally, a high electric field is applied across a drain and a source during its operation, and because carrying characteristic of electrons in the AlInAs layer is inferior than in as compared to the InGaAs layer, when a real space transfer occurs, it deteriorates the high-frequency characteristic of the FET.
The first HEMT of the prior art abovementioned uses as a channel a two-dimensional electron gas layer 8 in FIG. 1 (See FIG. 1) produced on a heterojunction interface between the undoped InP layer 2 and the n-AlInAs layer 3. This channel is formed within InP having a higher electron saturating speed than GaAs or InGaAs, thus producing a high-frequency device with upgraded performance. However, a limit has been placed against raising the electron gas density because the maximum current density of such HEMT is determined by the upper limit of two-dimensional electron gas density and the channel layer being in two-dimensional status. This results in an inability to produce a high-frequency device exhibiting a satisfactorily large output.
The fact as hereinbefore described applies also to the conventional cases of the second, third, and fourth HEMTs of the prior art. Electron gas density cannot be satisfactorily increased by the conventional second HEMT because the second HEMT uses as a channel a two-dimensional electron gas layer produced on a heterojunction interface between the undoped GaAs layer and the n-AlGaAs layer. Also, the electron gas density is not fully raised by the conventional third HEMT because the third HEMT uses as its channel a two-dimensional electron gas layer 19 (See FIG. 2) produced on a heterojunction interface between the undoped InGaAs layer 13 and the n-AlInAs layer 14. Likewise, the electron gas density is not fully raised by the conventional fourth HEMT because the fourth HEMT uses as its channel a two-dimensional electron gas layer produced on a heterojunction interface between the undoped InGaAs layer and the n-AlGaAs layer. Therefore high-frequency devices with a sufficiently high output have not been produced by the respective conventional HEMTs described above.
For other very high speed devices there has been developed, for example, a DMT (Doped-channel hereto MIS-FET) with the structures as in FIG. 7. A GaAs semiconductor substrate 21 has formed thereon an undoped GaAs layer 22, on which is formed an n.sup.+ -GaAs layer 23 that will be a channel. Further, on the n.sup.+ -GaAs layer 23 is formed an undoped AlGaAs layer 24 and an n.sup.+ -GaAs layer 25. A gate electrode 26 is formed in Schottky contact with an undoped AlGaAs layer 24 exposed at a recess, and ohmic electrodes 27 and 28 are formed on the n.sup.+ -GaAs layer
Because such DMT uses a high density and thinly formed n.sup.+ -GaAs layer 23 as a channel layer, a sufficiently large output can be obtained. The AlGaAs layer 24 thereabove is undoped to improve its Schottky withstanding voltage. However in this DMT, because the channel layer contains a large amount of impurities the travelling speed of electrons in the channel layer is lowered as compared with that of HEMT. As a result a high-frequency operational characteristic of DMT was inferior to that of HEMT.
Furthermore, as the DMT, as in the case of the second HEMT hereinbefore described, uses the heterojunction of the AlGaAs/GaAs, the electrons travelling in the n.sup.+ -GaAs layer 23 that will be a channel, are sometimes in real spatial transition to the AlGaAs layer disposed in the upper layer of the n.sup.+ -GaAs layer 23. Thus due to this real spatial transition there is a case where the high-frequency characteristic of the FET is further degraded.